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LECTURE 6 Chlor Alkali Industries – Soda Ash, Caustic Soda, Chlorine

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LECTURE 6 Chlor Alkali Industries – Soda Ash, Caustic Soda, Chlorine. Chapter 13 in Shreve’s Chemical Process Industies. Caustic soda, soda ash and chlorine Rank close to H 2 SO 4 and NH 3 in magnitude of $ value of use Lot of consumption in making other chemicals.

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lecture 6 chlor alkali industries soda ash caustic soda chlorine

LECTURE 6ChlorAlkali Industries – Soda Ash, Caustic Soda, Chlorine

Chapter 13 in Shreve’s Chemical Process Industies

chlor alkali industries

Caustic soda, soda ash and chlorine

  • Rank close to H2SO4 and NH3 in magnitude of $ value of use
  • Lot of consumption in making other chemicals.
  • Uses – Soaps, detergents, fibers and plastics, glass, petrochemicals, pulp n paper, fertilizer, explosives, solvents and other chemicals
Chlor Alkali Industries
caustic soda naoh

Previously made by Causticization of soda ash with lime

Na2CO3 + Ca(OH)2→ 2 NaOH + CaCO3

  • Only 10% NaOH solution obtained
  • Electrolysis of Brine – Most popular method adopted nowadays.
Caustic Soda – NaOH
caustic soda n a oh

Brittle white solid

  • Readily absorbs moisture and CO2 from air
  • Sold on basis of Na2O content
      • 76% Na2O equivalent to 98% NaOH
  • Uses – Soaps, textiles, chemicals, petroleum refining, etc.
Caustic Soda – NaOH
manufacture of naoh

Electrolysis of Brine

  • Chlorine at Anode; Hydrogen along with alkali hydroxide at cathode
  • Three types of cell exist:
    • Mercury Cell
    • Diaphragm Cell
    • Membrane Cell
  • Raw Materials

1. Brine (NaCl) 2. Electricity

Manufacture of NaOH
energy changes gibbs equation

Energy consumed in electrolysis is product of current flowing and potential of cell

  • Gibbs Helmholz equation represents the relation between electric energy and heat of reaction:
Energy Changes-- Gibbs equation
heat of reaction h

Found from heats of formation of the components of the overall reaction:

  • This reaction is broken down into following reactions for formation:

Net ∆H for the overall reaction results from

Heat of Reaction (∆H)
voltage efficiency

∆H is computed in Gibbs Helmholz equation to get E = 2.31 V

  • Voltage Efficiency = Epractical÷ETheoretical×100
  • Generally range from 60 – 75 %.
  • Faraday’s Law: 96,500C of electricity passing through a cell produce 1 gm.eq. of chemical reactions at each electrode
  • Actually higher – Side reactions
Voltage Efficiency
current efficiency and energy efficiency

Ratio of theoretical to actual current consumed is current efficiency (≈ 95-97%)

  • Current divided by area on which current acts is current density – high value desirable
  • Product of voltage efficiency and current efficiency is energy efficiency of cell
Current efficiency and Energy efficiency
decomposition efficiency

Ratio of equivalents produced in the cell to equivalents charged

  • Usually about 60 – 65 %.
  • Diaphragm cells have very high decomposition efficiencies
      • But encounter difficulties with migration of hydroxyl ions back to anode  formation of hypochlorite ion
      • At anode, OH- ions give
      • Oxygen formed reacts with graphite anode, decreasing its life
      • In Metal anodes, oxygen does not react.
Decomposition Efficiency
cell type

Previously mercury was most widely used

  • Health and environmental problems with mercury discharge in nearby waters
  • Improved designs of membrane cells and cheaper purification techniques have reduced cost and improved efficiencies
    • Dominate the field nowadays
Cell type
diaphragm cells
Diaphragm Cells
  • Contain a diaphragm made of asbestos fibers to separate anode from cathode
  • Allows ions to pass through by migration
  • Graphite anode and cast iron cathode
diaphragm cells1

Diaphragm Permits the construction of compact cells of lowered resistance as the electrodes can be placed close together

  • Diaphragms become clogged with use and should be replaced regularly
  • Diaphragm permits flow of brine from anode to cathode and thus greatly lessens side reactions
  • Cells with metal cathodes rarely get clogged diaphragms and operate for 1-2 years without requiring diaphragm replacements.
Diaphragm Cells
diaphragm cells advantages disadvantages

Major Advantage – Can run on dilute (20%), fairly impure brine

  • Dilute brine produces NaOH 11% (NaCl 15%)
  • Consumes lot of energy for evaporation
  • For 1 ton of 50% caustic need 2600 kg of water to be evaporated.
  • Some amount of Chloride ion remains and is highly objectionable to some industries (Rayon)
Diaphragm Cells– Advantages & Disadvantages
membrane cells
Membrane Cells
  • Use semipermeable membrane to separate anode and cathode compartments.
  • Separate compartments by porous chemically active plastic sheets; that allows sodium ions to pass but reject hydroxyl ions.
advantages of membrane cell

Purpose of membrane is to exclude OH- and Cl- ions from anode chamber

      • Thus making the product far lower in salt than that from a diaphragm cell
  • Membrane cells operate using more concentrated brine and produce purer, more concentrated product
      • (30-35% NaOH containing 50 ppm of NaCl)
  • Requires only 715 kg of water to be evaporated to produce 1 M ton of 50% NaOH
Advantages of Membrane Cell
advantages of membrane cell1

Because of difficulty and expense of concentration and purification, only large diaphragm cells are feasible

  • Membrane cells produce concNaOH
      • considerable saving in energy (Evaporation)
      • and saving in freight (operate to the point of caustic use)
  • Small, efficient units may cause a revolution in the distribution of the chlor-alkali industry, particularly if efficiencies remain high
Advantages of Membrane Cell
disadvantage of membrane cells

Membranes are more readily clogged than diaphragms, so some of savings are lost, bcos of necessity to pretreat the brine fed in order to remove Ca and Mg before electrolysis

Disadvantage of Membrane Cells
mercury cells

Operate differently than the other two

  • Cathode is a flowing pool of mercury; graphite anode
  • Electrolysis produces a mercury-sodium alloy (amalgam)
  • Amalgams is decomposed in a separate vessel as:

2Na.Hg + 2H2O → 2 NaOH + H2 + Hg

Mercury Cells
advantages and disadvantages of mercury

50% NaOH is produced with very low salt content (30 ppm)

  • No evaporation needed
  • Small loss of mercury to environment poses severe problems.
Advantages and Disadvantages of Mercury
unit operations and chemical conversions

Brine Purification

  • Brine Electrolysis
  • Evaporation and Salt Separation
  • Final Evaporation
  • Finishing of Caustic
  • Special Purification of Caustic
Unit Operations and Chemical Conversions
brine purification

Ca, Fe and Mg compounds plug the diaphragm

  • Precipitation with NaOH is commonly used to remove them
  • Addditional treatment with phosphates is required for membrane cells
  • Sulphates may be removed by BaCl2.
  • Brine is preheated with other streams to reduce energy requirement.
Brine Purification
brine electrolysis

3.0 – 4.5 V per cell is used; whichever method is adopted

  • Monopolar – Cells connected in parallel and low voltage applied to each cell
  • Bipolar – Cells are connected in series and high voltage applied
Brine Electrolysis
evaporation and salt separation

11 % NaOH (Diaphragm cells); 35% (Membrane Cells) are concentrated to 50% NaOH in multiple effect nickel tubed evaporators

  • Salt crystallizes out and recycled
  • Concentrated to 73% reduces shipping cost but greatly increases the shipping and unloading problems
  • High m.p of conc material makes steam-heated lines and steam heating of tank cars necessary.
  • Mp for 50% caustic 12°C; for 73%, 65°C.
Evaporation and Salt Separation
evaporation and salt separation1

Membrane cells produce more concentrated caustic than diaphragm cells

  • Less Evaporation or treatment needed (Membrane cell)
  • Mercury cells produce 50% solution, so no evaporation is needed
Evaporation and Salt Separation
final evaporation

Cooled and settled 50% caustic may be concentrated in a single-effect evaporator to 70 – 75% NaOH using steam at 500-600 kPa.

  • Strong caustic must be handled in steam-traced pipes to prevent solidification
  • It is run to finishing pots
  • Another method – Treating 50% Caustic solution with Ammonia
    • Countercurrent system in pressure vessels
    • Anhydrous crystals separate from resulting aq. ammonia
Final Evaporation
finishing of caustic

Dowtherm heated evaporators – removal of water

  • Product is pumped by a C.P that discharges the molten material into thin steel drums or into a flaking machine
Finishing of Caustic
special purification of caustic

Troublesome impurities in 50% caustic are Fe, NaCl and NaClO3.

  • Fe removed by treating caustic with 1% CaCO3 and filtration
  • NaCl and NaClO3 may be removed using aq. NH3
  • To further reduce salt content for some uses; caustic is cooled to 20°C as shown in following diagram
Special Purification of Caustic
chlorine and hydrogen

Dried Chlorine is compressed to 240 or 550 kPa

      • Lower pressure – rotary compressor
      • Larger capacities and Pressures – Centrifugal and non-lubricated reciprocating compressors
  • Heat of compression is removed and gas condensed
  • Liquid Cl is stored in small cylinders
  • Hydrogen used in making other compounds
      • With Cl HCl
      • Hydrogenation of fatty acids (Soap manufacture)
      • Ammonia
Chlorine and Hydrogen
soda ash manufacture

Soda Ash Manufacture

Sodium Carbonate

soda ash


    • Odourless/hygroscopic; alkaline in nature
    • Mp. 851 °C; M.wt = 106, Density @ 20 °C = 2.53 g/cm3;
  • Chemical
    • Thermal Decomposition at 1000 °C/200 Pa
    • Na2CO3 Na2O + CO2
    • Lethal dose = 4g/kg (rat); 15g/kg human
Soda Ash
uses of soda ash

Glass Industry

  • Water softening agent
  • Baking soda manufacture
  • Paper making
  • In Power generation to remove SO2 from flue gas
Uses of Soda Ash
manufacturing processes

Manufacturing processes

Le Blanc Process

Solvay Process

le blanc process

2 NaCl + H2SO4 Na2SO4 + 2 HCl

  • Na2SO4 + 2C  Na2S + 2 CO2
  • Na2S + CaCO3 Na2CO3 + CaS
  • Disadvantages
    • Solid Phase
    • Amount of energy
    • CaS pollutant
Le Blanc Process

Brine (NaCl)

Ammoniated Brine





Limestone CaCO3



Lime in


Carbonating Tower


Ammonia Recovery





Lime Slaker


Waste by product CaCl2

300 °C



  • Food additive
  • 2. Electrolyte
  • 3. Dehydrating agent

Solvay Tower

      • 2 NH3 + CO2 + H2O  (NH4)2CO3 (exothermic)
      • (NH4)2CO3 + CO2 + H2O  2 NH4HCO3
      • NH4HCO3 + NaCl  NaHCO3 + NH4Cl2

Middle of Carbonator

  • Lime Kiln
      • CaCO3  CaO + CO2
      • CaO + H2O  Ca(OH)2
  • Calciner
      • 2 NaHCO3  Na2CO3 + CO2 + H2O
  • Ammonia Recovery
      • 2 NH4Cl + Ca(OH)2  CaCl2 + 2 NH3 + 2 H2O
manufacturing steps

Brine Preparation

  • Ammonia Absorption
  • Precipitation of bicarbonate
  • Filtration of bicarbonate
  • Calcination of bicarbonate
  • Recovery of Ammonia
Manufacturing Steps
solvay process2
Solvay Process
  • NH3 Absorber
    • Counter current flow; Baffles tray
    • Cooler to remove heat of solution
    • Slightly less than atm pressure
    • Made of Cast iron
    • At exit; NaCl = 260 g/l; NH3 = 80-90 kg/m3; CO2 = 40-50 kg/m3
  • Carbonator
    • 6 -9 in number; 20-30 m in height
    • Exothermic reaction 60 °C
    • To reduce solubility of NaHCO3 use cooler at bottom @ 30 °C
    • Vacuum Rotary filter at bottom